1. Field of the Invention
The present invention relates to a method and a device for vibration control, and more specifically a method, a device and a tool holder for vibration control in cutting.
2. Background Art
In cutting, such as turning, drilling, milling or planing, dynamic motion arises between the tool and the workpiece. The motion is largely due to the fact that the chip-forming process, i.e. the removal of the generally relatively hard material from the workpiece, results in dynamic excitation of the tool, especially the tool holder. The dynamic excitation results in a dynamic motion, in the form of, for instance, elastic bending or torsion, of the tool and the tool holder. The chip-forming process is largely stochastic and the excitation results in tool vibrations and noise. In addition to thus causing problems in the working environment, the dynamic motion also affects the evenness of the surface of the workpiece and the service life of the tool.
It is therefore important to reduce the dynamic motion as far as possible. It is known that the vibration problem is closely connected with the dynamic stiffness in the construction of the machine and the material of the workpiece. It has therefore been possible to reduce the problem to some extent by designing the construction of the machine in a manner that increases the dynamic stiffness.
An important part of the construction is the actual tool holder. The cutting tool, for instance turning insert (or tooth), milling teeth or drilling teeth, is rigidly supported by the tool holder. Consequently the vibrations arising between the cutting edge and the workpiece are transferred almost completely to the tool holder. In many cases, it is the lack of dynamic stiffness of the tool holder that is a main problem.
Efforts have therefore recently been made to increase the dynamic stiffness of the actual tool holder by means of active technique in order to control the response of the tool. This means that active control of the tool vibrations is applied.
The active control comprises the introduction of secondary vibrations, or countervibrations, in the tool by means of a secondary source which is often called actuator. The actuator is operated in such manner that the countervibrations interfere destructively with the tool vibrations.
U.S. Pat. No. 4,409,659 discloses an example of such a control unit. An ultrasonic actuator is arranged on the tool holder and produces countervibrations in the tool. The operating current of the actuator is controlled according to physical parameters that are measured and by means of the work of the actuator are kept within defined limits. This construction is unwieldy since the actuator is a comparatively large component which must be mounted on a suitable surface of the tool holder. Moreover, the directive efficiency is not quite distinct.
JP-63,180,401 discloses a very different solution where the actuator is built into the tool holder which holds a turning insert. A laterally extending through hole which is rectangular in cross-section is formed in the tool holder. A piezoelectric actuator, in series with a load detector, is fixed between the walls that define the hole in the longitudinal direction of the tool holder. The load detector detects the vibrations and is used by a control unit to generate, via the actuator, countervibrations which reduce the dynamic motion. This construction necessitates a considerable modification of the tool holder and indicates at the same time that the designer has not been aware of the essence of the excitation process. In fact, the modification counteracts the purpose of the construction by reducing the stiffness of the tool holder in the most important directions, above all vertically, which in itself causes a greater vibration problem, or alternatively means that the dimensions of the tool holder must be increased significantly in order to maintain the stiffness. During turning, the rotating workpiece produces a downwardly directed force on the cutting edge. When the cutting edge offers resistance, material is broken away from the workpiece. In this context, most of the vibrations arise. In JP-63,180,401, one imagines that the surface of the workpiece is uneven (wave-like) and thus mainly excites the tool holder in its longitudinal direction. Via the actuator, one generates an oscillation in opposition towards the wave pattern and thus obtains a constant cutting depth.
There is thus a need for a solution which controls the most essential vibrations in cutting, such as turning, milling, drilling or planing, and which causes a minimum of negative effects, such as bulky projections of dynamically weakening modifications, and still has a good effect.